How to make update rapidly?

Hi,Every body;
   I have a table contains 100,000 rows, and has a primary key(int).
  Now ,I need to execute sql command like "update .......... where id=*"(id is primary key).
  I expect execute 1200-1600 sqlcommands per second(1200-1600/s).
  In test,when the id increase by degrees in sqlcommands, then I can reach the speed(1600/s);
  But in fact , the id  in sqlcommands  is out of rule, then the speed is very slow, just 100/s.
  what can i do? can you help me ?